1. Field of the Invention
The invention relates to a method for manufacturing an inflatable closing plug for pipes, in particular comprising a balloon-like element of elastic material. The invention further relates to inflatable closing plugs of improved design, to methods of closing a pipe with the plug and to an apparatus for manufacturing an inflatable closing plug.
2. Description of the Related Art
Inflatable plugs for closing pipes, for example gas pipes, are known. In use they are inserted into an opening in a pipe while deflated, whereafter the balloon-like element is filled with a pressurized fluid so that the balloon inflates to a widened state and closes the passage of the pipe. The pressurized fluid may be compressible or incompressible, for example compressed air, nitrogen, hydraulic oil or water. In order that the balloon is able to exert sufficient pressure on the inner wall of the gas pipe, the balloon has a limited freedom of axial extension. To this end, a separate sheath of woven material is arranged around the balloon. This separate sheath has limited extensibility so that, when the balloon is inflated, it reduces or substantially prevents extension of the balloon axially along the pipe. The inner-pressure of the balloon is instead applied radially against the inner wall of the pipe so that an adequate closing pressure can be obtained. The sheath may also be used to limit radial expansion.
Manufacture of a balloon with a separate sheath therearound is quite time-consuming. Additionally the working diameters of such balloons are limited, meaning they are usable to close only a small range of pipe diameters. This disadvantageously means that an extensive variety of balloons with different nominal working diameters must be used and held in stock. Examples of inflatable plugs having balloons with separate sheaths are known. WO2006/044483 discusses an inflatable, expandable bag, constructed by providing a flexible material having a gas proof liner and a woven protective layer; forming a bag from the material; placing a sleeve over the material; and crimping the sleeve onto the material. EP0664417 discusses an expandable sealing element comprising a balloon provided with a stiffening means to counteract the expansion of the balloon; the stiffening element comprises a separate sheath of knitted material. EP0583819 discusses a device for closing the passage in a pipe, provided with an inflatable body of elastic material and a sleeve of less elastic material that envelopes the inflatable body. The sleeve is formed from a cylindrical part taken in at the end with slanting pleats.
Plugs in which the balloon-like element is provided with reinforcement lines in the outer surface thereof are also known. The lines may be anchored to end pieces, for example, as discussed in WO2005/003617, where a plurality of concentric layers of reinforcing lengths are provided in the thickness of the balloon's wall. In another example as discussed in US2005/0229985, an inflatable, flexible device is made from a latex body provided with a fibre or mesh reinforcing structure supplied as a thread or line winding as a pre-knitted mesh or randomly applied fibres. The manner of manufacturing such a balloon-like element is likewise time-consuming and costly.
Useful methods of manufacturing inflatable closing plugs for pipes are discussed in prior published patent U.S. Pat. No. 5,477,886, the contents of which are hereby included by reference. Generally in the discussed method, a balloon-like element is inflated to a predetermined pressure,                a line or a group of lines with low elasticity is laid from the one end of the balloon to the other end of the balloon, and        the line or lines is or are adhered to the surface of the balloon e.g. with latex.        
According to more specific embodiments discussed therein it is possible a) to spray the line onto the surface simultaneously with the latex; b) to spin lines from fibres prior to spraying, adhere them with latex and subsequently arrange them on the surface of the balloon; c) to draw a single line from a supply and wind it around the balloon ends using a winding arm so that a large number of line windings can be arranged on the balloon surface; or d) a plurality of reinforcing lines are applied to a balloon like element by making use of an annular element having a plurality of circumferentially oriented guide holes for multiple lines, the balloon is passed through the annular element and a plurality of axial lines are laid on its surface and fastened by rings at either extremity.
Of these methods, the method of winding a single line around the balloon has been found to be the most effective and useful in practice. However, there exist a number of problems with this method. In order to adequately reinforce the balloon a sufficient number of line windings must be provided around the balloon. Depending upon the expected external and internal pressures the plug is to be subjected to and upon the size of the balloon, the number of windings may typically range from 100 to several hundred-thousand, with some balloons having 400,000 windings; and more common balloons having in the range of 10,000 to 20,000 windings. The application of so many windings is time consuming and thus costly.
Additionally, according to the prior process, only one type of line can be added as the reinforcing material at any one time. This limits the balloons to having only one type of line or to having distinct layers of different line. The provision of different line types requires a halt in the process to change the line, and is also limiting because the process is unable to provide a blend of lines within a single layer of windings. In particular in some applications it may be desirable to provide conductive materials (comprising metals or graphite) as part of the sheath This may require the use of special materials for the lines, which may be expensive or have compromised properties as a result of the inclusion of conductive materials. It may also or alternatively require repeated line changes.
Known plugs also suffer from the use of large quantities of adhesive used to adhere the lines to the surface of the balloon-like element. A preferred adhesive material is latex and the use of excess material leads to increased costs. It may also lead to the provision of plugs having unnecessarily thick and non-supple walls making them difficult to manipulate into and out of position in a pipe. It also increases production time because of drying requirements.
The use of excess adhesive may also lead to an uneven, or exaggeratedly uneven, spread of the adhesive over the surface of the balloon-like element. This is particularly the case with certain existing winding procedures which, for large diameter balloons, are carried out with the balloon suspended vertically. The latex or other adhesive used has a tendency to flow towards the lowest point of the balloon leading to particularly thick walling in that area. This again makes the plug less supple, and also may lead to poorly shaped plugs, for example pear shaped plugs because of a build up of adhesive at one end.
A further problem associated with known plugs is that the finished surface of the plug is often rough where the wound reinforcing line has bunched into troughs and furrows on the balloon surface, instead of lying evenly spread. This can increase the friction coefficient of the plug's surface and result in insertion and removal difficulties.
It has also been found with known plugs that, as a result of the manufacturing processes, a build-up of line occurs at the balloon ends or poles where the wound line overlaps. In particular on high-pressure balloons having large numbers of windings, the increased thickness at the poles makes the plug less flexible and difficult to collapse to a small diameter. The deflated plug may be difficult to manipulate through pipe inlets, outlets, saddles etc.
As mentioned above, U.S. Pat. No. 5,477,886 also discusses a method of simultaneously applying multiple lines to a balloon. However, the number of line lengths that can be laid onto the balloon surface is highly restricted by the number of eyelets that can be provided on the annular member so that adequate reinforcement is not achievable.
Methods of winding or wrapping objects in which a band or strip is used are also well known. By winding a strip, a larger area can be covered with a given number of windings than when winding with a line. Winding a flat strip onto a three dimensionally curved surface is however problematic unless the strip is elastically deformable. If the strip is substantially inextensible it will not lie flat on a spherical surface and ridges and pleats may appear. Under tension, the distribution of forces within the strip is unlikely to be ideal, making such a procedure unsuitable for the manufacture of inflatable plugs.
The invention has for its object to obviate at least some of the above stated drawbacks.